This invention relates generally to the field of nozzles for producing a water spray, and more particularly to a nozzle for use in papermaking machinery that produces a water spray using air atomization for remoisturizing a web of paper under manufacture.
Conventional papermaking machinery for producing a continuous sheet of paper includes equipment to set the sheet properties of the paper as it is being manufactured. Generally, on-line measurements of sheet properties, such as thickness, moisture, gloss or smoothness are made by scanning sensors that travel back and forth across the width of the sheet of paper in the cross-machine direction (CD). The scanning sensors are located downstream of actuators that are controlled to adjust the sheet properties. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. In practice, the actuators provide generally independent adjustment at adjacent cross-directional locations of the sheet, normally referred to as slices or profile zones.
One of the more basic operations on a paper machine is control of the cross-direction moisture profile by remoisturizing with water sprays administered by spray nozzles. By applying water to the drier areas of a sheet, a uniform CD moisture profile can be created.
There are two predominant remoisturizing systems in use today that both rely on water spray nozzles positioned along the cross-machine direction. These systems are distinguished by the different nozzles that are usedxe2x80x94air atomized and hydraulic atomized nozzles. Typically, air atomized nozzles produce a hollow or solid conical spray that delivers water in a generally circular pattern. Air pressure or the air/water ratio are varied to adjust between the hollow and solid spray patterns. Hydraulic nozzles produce a flat fan spray that delivers water in a generally elliptical pattern. Conventionally, both systems employ 4-bit logic so that 16 different water flows are possible per CD profile zone by various on/off combinations of the nozzle control solenoids.
Air atomized systems generally comprise a boom that extends in the cross-machine direction equipped with a single nozzle per profile zone with 4 off-machine solenoids per nozzle that provide the required 16 water flow rates. A 100 mm CD profile zone width is standard.
Hydraulic atomized systems also generally consist of a boom extending in the cross-machine direction, however, each profile zone is conventionally defined by 4 nozzles producing discrete, flat fan sprays that overlap without intersecting. CD moisture profile zone width is variable down to 50 mm with 100 mm being the most common spacing.
Applicant has developed an air atomizing nozzle that has advantages over existing nozzles. The present invention provides a nozzle assembly for converting liquid from a liquid source into a spray flow using air from an air pressure source to atomize the liquid comprising:
a housing having an air inlet for connection to the air pressure source, a liquid inlet for connection to the liquid source, and an outlet;
at least one first air passage in the housing for receiving air from the air inlet, the at least one first air passage communicating with the outlet and being configured to generate a swirled air stream;
a second air passage in the housing for receiving air from the air inlet, the second air passage communicating with the outlet and being configured to generate a linear air stream;
a liquid passage for receiving liquid from the liquid inlet, the liquid passage contracting to a reduced cross-sectional area adjacent the outlet; and
the first and second air passages and the liquid passage terminating in a common atomization zone adjacent the housing outlet where liquid flowing through the liquid passage is atomized into a spray pattern.
The nozzle of the present invention has a unique construction with multiple air passages and a constricted water passage that relies on substantially constant low air pressure in the range of 2.5 to 4.0 psi to achieve adequate spray characteristics over a broad range of spray flow rates. Prior art air atomization nozzles generally require high capacity compressed air systems operating in the region of 15 p.s.i.
Instead of relying on four-bit logic control limited to 16 discrete flow steps, the nozzles of the present invention are sufficiently flexible that they can provide an infinite range of water flows between current minimum and maximum industrial flow rates by adjusting the water flow rate to the nozzle with good atomization over the entire range of flows. This allows the nozzles to be used for CD moisture control for all grades of paper from fine specialty paper to heavier board. As well, control solenoids can be reduced from four to one per zone as the nozzles are not limited to four-bit logic. The nozzles of the present invention can also be operated using the four-bit logic control of the prior art.
In a preferred embodiment, the nozzles of the present invention are of a modular design that allows different inserts with different spray characteristics to be used depending on the paper being manufactured.
The nozzles of the present invention tend to have a uniform CD flow distribution pattern resulting in much flatter zone spray profiles.
The spray patterns produced by the nozzle of the present invention tend to be smaller while maintaining good atomization with the result that the minimum profile zone spacing using the present nozzle can be half of the current minimum spacing. The smaller spray width also provides more consistent discrete zone control.
The nozzles of the present invention use larger orifices than prior art designs which reduces clogging problems.
In view of the above-described advantages, the nozzle design of the present invention tends to be easier to maintain resulting in lower maintenance costs and less overall cost.